punk on Nostr: Never done a spy arc myself, but my thought is to really lay on the tension between ...

Never done a spy arc myself, but my thought is to really lay on the tension between your players by giving them each secrets and secret goals that conflict with each other and potential outcomes.
Have a coded message they must solve. No too hard / or not essential for the story. Maybe just for some extra loot.
Lastly, steal all types of stuff from your favorite spy movies or books…have an old artificer npc outfit your party with custom spy gadgets. Or a friendly npc seems helpful but is secretly working for the enemy.
Good luck!